THE 2ND HALF OF THE CYSTIC-FIBROSIS TRANSMEMBRANE CONDUCTANCE REGULATOR FORMS A FUNCTIONAL CHLORIDE CHANNEL

The cystic fibrosis transmembrane conductance regulator (CFTR) consist s of two transmembrane domains (TMDs), TMD1 and TMD2, two cytoplasmic nucleotide binding domains (NBDs), NBD1 and NBD2, and a regulatory dom ain. To elucidate the complex function of the CFTR, deletion construct s encompassing the second half of the CFTR distal to the first transme mbrane domain were expressed in Xenopus oocytes and IB3 cells (a cysti c fibrosis cell line). Constructs containing the regulatory domain, th e second transmembrane domain, and the second nucleotide binding domai n formed constitutively active channels, which were further stimulated upon the addition of cAMP. On the other hand, a construct encompassin g the second transmembrane domain and the second nucleotide binding do main was stimulated to a small but noticeable extent upon the addition of cAMP, The selectivity of the second-half construct was the same fo r iodide and chloride, in contrast to the selectivity of wild-type CFT R, which is Cl- > I-. However, both constructs displayed single-channe l conductances that were significantly smaller than those displayed by the first half of the CFTR, We conclude that regions of the second tr ansmembrane domain may contribute to the overall channel of the pore, although the first half of the CFTR may confer its selectivity.